| Anti-mise en Offset
Explanation:
Guesswork for the moment, but I'll try to refine it, or maybe it will give s.o. else ideas.
Sounds to me as if they are talking about an OTC (overhead travelling crane), i.e. a crossbeam mounted on a trolley at each end, the trolleys travelling on two elevated longidinal beams or runways. If the ratio of beam span to trolley length (rapport de la portée sur l'empattement ) is high, there is a risk of the main beam getting "diagonal" to the runways (instead of being perpendicular thereto), i.e. the trolleys are "offset" with respect to oneanother, instead of being precisely opposite each other.
Which means of course that the crane is going "sideways" to a certain extent. I don't think anything with "crab" will work since "crabbing" is the sideways movement of the hoist along the main beam, as opposed to the perpendicular movement of the entire crane along its runways.
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Note added at 7 hrs 57 mins (2004-12-04 22:51:26 GMT)
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GOT IT : anti-skewing system
CRANE ANTI-SKEW & POSITIONING CONTROL
Application
Long Span Crane & Gantry Crane System
Overhead cranes frequently skew due to poor rail conditions, uneven wheel wear, wheel slippage or unequal load conditions when the trolley is operating at one end of the crane bridge. Skew is a common problem on long span bridge cranes and gantry cranes with single or multiple drives.
Problem
Skewing of the crane can cause excessive wheel wear and stress, especially to the wheel flanges. It can also produce horizontal forces at right angles to the rail which can result in unusual stresses to the crane runway beams and building structure. Polyurethane wheels on trackless gantry cranes tend to wear more rapidly than steel wheels. This often results in different diameter driving wheels which subsequently cause the crane to skew. Crane skew also makes it more difficult to spot loads accurately.
Solutions
Mechanical methods used to reduce skew in the past, include:
increasing the crane end truck wheel base,
decreasing the clearance between the wheel flange and rail, or
adding side guide rollers.
An improved method is to use low slip motors/drives on each end truck, with separate Electromotive IMPULSE® adjustable frequency crane controls, providing controlled speed and torque control. This also allows reduced wheel and rail wear. For example, if the east end of the crane bridge is detected to be traveling ahead of the west end, the east end motor will be driven slower to allow the west end to catch up.
The amount of allowable skew is dependent upon the application requirements. The above methods can be used individually or in combination to reduce or eliminate crane skew. Electromotive Systems anti-skew control improves crane alignment thus reducing wheel flange wear and improving load spotting ability.
Technical Description
Mechanical Crane Skew is shown on the following diagram.
(clearly shows \"diagonalism\" of crane relative to tracks)
Several methods are available to control crane skew:
1. If the crane is equipped with independent motor drives on each end truck, the motors will \'slip\' keeping the crane in line. With crowned or special wheel configurations, the crane tends to be self centering. Using separate low slip motors and Electromotive IMPULSE adjustable frequency crane controls for motor control, helps reduce the results of crane skew. In these cases, the amount of skew is still dependent upon the mechanical clearance between the wheel flange or guide roller and the rail. Although wheel and rail wear is reduced, spotting accuracy is still affected.
2. On those crane systems, where spotting accuracy is critical and where a PLC (programmable logic controller) is used for automatic crane operation (or can be added), a more positive method can be applied. Distance detection sensors are mounted on the end trucks at each end of the crane. These sensors measure the distance to fixed \'targets\' mounted with direct reference to the final spotting positions. The distance sensors are connected to the PLC which in turn is used to control the individual IMPULSE adjustable frequency crane controls controlling individual motors on each end truck. The PLC automatically adjusts the output signals to each crane control for any distance deviations detected between the two sensors. Dependent upon the accuracy of the distance detectors, skew can be eliminated during travel and at final positioning.
[http://www.electromotive.com/elecsolutionsarticle-craneantis...]
The exclusive Demag EndTruck/Girder connection utilizes a friction connection. This friction connection includes high torque bolts in combination with precision washers that result in a controlled overall dimension accuracy within a tolerance of 2 millimeters for the total dimension of the endtruck/girder assembly. The main benefit of this design is reduced crane skewing through improved crane squareness and span accuracy. Additional benefits include reduced maintenance due to reduced wheel wear and interchangeable replacement components.
[http://www.omhilinc.com/id2.html]
SKEWING FORCES: Lateral forces on the bridge truck wheels caused by the bridge girders not running perpendicular to the runways. Some normal skewing occurs in all bridges.
(where \"bridge\" = \"bridge crane\")
[http://www.osborn-ind.com/definitions.asp]
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Note added at 8 hrs 50 mins (2004-12-04 23:44:48 GMT)
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Just checked that the French does indeed mean this, and it does: see lengthy but clear explanation at http://www.paris.ensam.fr/docpfe/M2266.pdf
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Note added at 2 days 10 hrs 15 mins (2004-12-07 01:10:10 GMT)
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For Jane:
If you don\'t like it, don\'t read it. My answer is for the Asker after all. My \"mights\", \"could bes\", and \"suspects\" are in the hope that the Asker, who has the fuller context, will be able to put two and two together, assuming my assumptions match reality.**I** don\'t have the full context, so cannot assert as confidently as some might that I am 100% right, but I can suggest the most likely probabilities, as I see them, based as much, if not more, on my knowledge of the field than on the words alone.
| Bourth (X)
Local time: 15:34
Native speaker of:  English
PRO pts in category: 447
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